I am broadly interested in the following question: What makes a drug addict different from a recreational user? To approach this question, I use rodent behavioral and neurophysiological models. Using techniques such as intravenous self-administration, voluntary ethanol drinking, and behavioral models of anxiety, reward, and aversive effects of drugs of abuse, we have begun to identify some factors that differentiate casual drug users from drug addicts. Ongoing research is aimed at further clarifying these factors and elucidating physiological mechanisms.